This invention relates to the field of purifying gaseous streams contaminated by CO.sub.2 and H.sub.2 S and possible other impurities, and generally involving the step of reacting SO.sub.2 with H.sub.2 S to form elemental sulfur.
In the process industries, it is frequently necessary to separate sour gases, understood to mean essentially CO.sub.2, H.sub.2 S and mercaptans, from a given gaseous stream. These compounds, which, due to their corrosive and catalyst-impairing properties, or for other reasons, must generally be maintained out-of-contact with downstream processing units. CO.sub.2 and H.sub.2 S are the predominant sour gases found in gaseous process streams, such as, in particular, natural gas or cracked gases.
Since in the separation of sour gases from raw gases, H.sub.2 S, in contrast to CO.sub.2, must, due to its toxicity, not be discharged into the environment, or at least only in extremely low concentrations, the H.sub.2 S component is conventionally processed into elemental sulfur in a downstream facility. However, inasmuch as the CO.sub.2, relatively harmless to the environment, is merely an undesirable inert throughput in a sulfur-producing plant, scrubbing operations selective for H.sub.2 S are utilized to a large extent for removing these sour gases from gaseous streams, thereby permitting the size of the sulfur-producing plant to be substantially decreased. At the same time a sufficiently high H.sub.2 S concentration is achieved which ensures a stably burning flame in the Claus burner. In these scrubbing processes, a residual gaseous stream free of H.sub.2 S and containing carbon dioxide can be obtained in the regeneration of the scrubbing medium, as well as, besides, a gaseous stream rich in hydrogen sulfide and containing part of the separated CO.sub.2. This H.sub.2 S-free residual gas stream can be put to its final use, e.g. downstream processing units. Optionally, pure CO.sub.2 for other usages can also be isolated from this stream.
Chemical as well as physical scrubbing methods have been developed for the selective separation. The physical scrubbing processes have been employed preferentially for many years to a large extent, particularly in the purification of gaseous streams relatively extensively contaminated by CO.sub.2. The scrubbing liquids utilized in this connection dissolve the sour gas components without chemical reaction and can be regenerated from these components by expansion, heating and/or distillation.
The H.sub.2 S-enriched residual gas fraction obtained in the scrubbing step selective for H.sub.2 S contains, depending on the hydrogen sulfide content of the gaseous stream to be purified, generally between 10 and 85 mol-% of H.sub.2 S and is processed to elemental sulfur, for example, in a sulfur-production plant based on the Claus reaction EQU 2H.sub.2 S+SO.sub.2 .fwdarw.3/xS.sub.x +2H.sub.2 O+.DELTA.H
However, the conversion to elemental sulfur is never complete; rather, a residual gas containing H.sub.2 S and SO.sub.2 is obtained in all cases. Although most of the sulfur compounds separated from the gaseous stream are processed to elemental sulfur, the residual gas mostly still contains sulfur compounds in such concentrations as to prohibit their discharge into the atmosphere. For further details of the Claus process, reference is invited to the extensive literature on the subject, e.g., Kohl and Riesenfeld, Gas Purification, 3rd Edition, 1979, Gulf Publishing Co., Houston, Tex., incorporated by reference herein, especially pages 410-421, and "Sour Gas Processing and Sulfur Recovery", The Petroleum Publishing Company, Tulsa, Okla., 1979, page 65ff.
For the further purification of the Claus process residual gas, many systems have been suggested, including one wherein the residual gas, after cooling and sulfur separation, is subjected to oxidation in the presence of air and combustion gas. In this step, all hydrocarbons that may be present are oxidized to CO.sub.2 and water, and all sulfur compounds are essentially oxidized to SO.sub.2. The resultant gas is cooled and scrubbed in an absorber with triethanolamine which is selective for SO.sub.2. The loaded solvent is freed in a stripper from SO.sub.2 by lowering the pressure; the SO.sub.2, saturated with water, is recycled into the raw gas stream upstream of the Claus reactor (Hydrocarbon Processing, July 1979, pp. 197-200).
However, this process has the serious disadvantage that the recycled SO.sub.2, in all cases, also contains several ppm of the amine which, in case of incomplete combustion in the Claus installation, reacts and fouls the catalyst. Considering long-term operation, the heat exchangers can also be plugged up by the formation of carbamates and other ammonium compounds which, especially on the relatively cool parts of the installation, lead to insoluble deposits. Therefore, within a time frame that maybe as short as 2 months, the catalysts must be removed and the heat exchangers must be made serviceable again, with great expenditures in energy, time and money.